1. Field of the Invention
This invention relates to zoom lenses, and more particularly to such zoom lenses suited for video cameras.
2. Description of the Prior Art
Since the image pickup element of a video camera has a low sensitivity, the speed of the camera's objective lens should be fast and, in order to assist in reducing the size and weight of the camera as a whole, the physical length of the objective lens should be short. Another demand on the objective lens arises from the fact that a video camera may employ a stripe filter with single-tube type color image pickup device, or CCD imaging plate as the image pickup element. In such a camera deviation of the incident light on the stripe filter from an almost perpendicular direction results in a mixture of colors. Thus it is necessary to construct the objective lens in the form of a telecentric optical system.
In the present state of the art of zoom lenses, shortening of the entire lens system in the longitudinal direction is achieved by reducing the total axial movement of the variator. A most effective method for such a purpose is strengthening the refractive power of each of the lens members forming the variator. However the increase in the refractive power calls for imparting a stronger curvature to each of the lens surfaces in the variator. In zoom lenses this results in extreme difficulty of stabilizing good correction of spherical aberration, coma and distortion throughout the entire zooming range.
That is, when the prior art rules of zoom lens constructions are applied to objective lenses for video cameras to fulfill the above-stated requirements, high grade imaging performance becomes very difficult to achieve. Poor correction of spherical aberration, coma and distortion due to reduction of size, decreases the contrast and deforms the image.
An example of a zoom lens usable with a video camera is the one disclosed in Japanese Laid-Open Patent Application No. SHO 57-19707 of Feb. 2, 1982.